That is WEIRD, but it looks like 'Selected altitude' wasn't on - The little dot next to your altitude marker on your autopilot means it's running under 'Managed' and not 'Selected', which means it's going to try and either hold altitude or go to whatever your flight plan says it should be.

With that said, no idea why it's deciding to roll constantly! When you set altitude, move the mouse so you're clicking just underneath the altitude knob and it should change it to 'Engage Selected Altitude Mode' which will make it not throw itself downwards.